Until the 1950s, a non-crosslinked polyethylene resin was mainly used as an insulating material of a power cable, but the non-crosslinked polyethylene resin had limitations in long-term heat resistance and durability. Since a crosslinking technology for improving long-term heat resistance and durability of polyethylene was developed by Union Carbide (USA) in the 1950s, currently, crosslinked polyethylene has been mainly used in the power cable.
As a method of crosslinking polyethylene, there are a method of crosslinking polyethylene by a chemical reaction using organic peroxide or silane (U.S. Pat. No. 6,284,178, Sep. 4, 2011), a method of crosslinking polyethylene using an electron beam (U.S. Pat. No. 4,426,497, Jan. 17, 1984), and the like. Recently, the method of crosslinking polyethylene using the organic peroxide has been widely used in a cable industry.
Since the crosslinked polyethylene resin is a thermoset resin, the polyethylene resin has excellent heat resistance, chemical resistance, and electrical properties.
However, since the crosslinked polyethylene resin is the thermoset resin and the thermoset resin is not recyclable, it is difficult to dispose of the waste polyethylene resin, thereby causing environmental pollution. Therefore, there is a demand for an eco-friendly non-crosslinked type thermoplastic polyethylene resin, but heat resistant of the non-crosslinked type thermoplastic polyethylene resin was significantly insufficient as compared with the crosslinked polyethylene resin, such that there was a limitation in using the non-crosslinked polyethylene as the insulator of the power cable.
Nevertheless, in some countries such as France, and the like in Europe, the thermoplastic polyethylene resin is used as the insulator of the power cable in order to protect the environment and avoid the above-mentioned disadvantages of the crosslinked polyethylene resin.
In a process of producing power cables with the polyethylene crosslinked by the organic peroxide, a crosslinking process is essential. At the time of the crosslinking, high-pressure and high-temperature conditions are required, and productivity thereof is significantly low, such that a difference in a crosslinking degree may be generated even with a slight change in the process condition, thereby deteriorating uniformity of the product.
In addition, during the crosslinking process, the organic peroxide is decomposed by heat to form radicals, thereby completing the crosslinking reaction. At this time, cumyl alcohol, methane, and the like, are generated as by-products of the crosslinking reaction to form bubbles in the insulator. In order to remove this bubble, high pressure of 5 atm or more should be applied. Bubbles that are not removed may cause breakage of the insulator of the power cable.
Against this backdrop, research into a non-crosslinked polyethylene resin as the insulating material of the power cable was disclosed in Korean Patent Laid-Open Publication No. 10-2010-0106871 (Dec. 4, 2010). However, at the time of actually processing the non-crosslinked polyethylene resin, processability is poor due to low shear thinning of the resin, such that a processability defect may be generated.
In addition, AC dielectric breakdown strength, space charge distribution, and water tree resistance are poor, such that performance as the insulator may be deteriorated.